What happens if you don't commit a transaction to a database (say, SQL Server)?

Question

Suppose I have a query:

begin tran
-- some other sql code

And then I forget to commit or roll back.

If another client tries to execute a query, what would happen?

Answer 1

As long as you don't COMMIT or ROLLBACK a transaction, it's still "running" and potentially holding locks.

If your client (application or user) closes the connection to the database before committing, any still running transactions will be rolled back and terminated.

Answer 2

You can actually try this yourself, that should help you get a feel for how this works.

Open two windows (tabs) in management studio, each of them will have it's own connection to sql.

Now you can begin a transaction in one window, do some stuff like insert/update/delete, but not yet commit. then in the other window you can see how the database looks from outside the transaction. Depending on the isolation level, the table may be locked until the first window is committed, or you might (not) see what the other transaction has done so far, etc.

Play around with the different isolation levels and no lock hint to see how they affect the results.

Also see what happens when you throw an error in the transaction.

It's very important to understand how all this stuff works or you will be stumped by what sql does, many a time.

Have fun! GJ.

Answer 3

Transactions are intended to run completely or not at all. The only way to complete a transaction is to commit, any other way will result in a rollback.

Therefore, if you begin and then not commit, it will be rolled back on connection close (as the transaction was broken off without marking as complete).

Answer 4

depends on the isolation level of the incomming transaction.

Sql transaction isolation explained

Answer 5

When you open a transaction nothing gets locked by itself. But if you execute some queries inside that transaction, depending on the isolation level, some rows, tables or pages get locked so it will affect other queries that try to access them from other transactions.

Answer 6

Example for Transaction

begin tran tt

Your sql statements

if error occurred rollback tran tt else commit tran tt

As long as you have not executed commit tran tt , data will not be changed

Answer 7

Any uncomitted transaction will leave the server locked and other queries won't execute on the server. You either need to rollback the transaction or commit it. Closing out of SSMS will also terminate the transaction which will allow other queries to execute.

Answer 8

In addition to the potential locking problems you might cause you will also find that your transaction logs begin to grow as they can not be truncated past the minimum LSN for an active transaction and if you are using snapshot isolation your version store in tempdb will grow for similar reasons.

You can use dbcc opentran to see details of the oldest open transaction.

Answer 9

I really forget to commit a transaction. I have a query like codes below.

This stored procedure is called by .Net. When I test the function in .Net application, the exception will be captured in .Net application.

Exception message like below:

Transaction count after EXECUTE indicates a mismatching number of BEGIN and COMMIT statements. Previous count = 0, current count = 1.

When I realize the mistake, I have tried many times, both in .Net application and SQL Server Management Studio (2018). (In SSMS, the output statement will successfully output the result in Results tab, but shows the error message in Messages tab.)

Then I find the tables used in this transaction are locked. When I only select top 1000 without order desc, it can select the result. But when I select top 1000 with order desc, it will be running for a long time.

When I close the .Net application, the transaction was not committed (based on the data not changed in the transaction).

When I close the EXEC ... tab (which execute the forged commit query), SSMS will pop a warning window:

There are uncommitted transactions. Do you wish to commit these transactions?

I have tested the both the Yes and No choices.

If I click Yes, the transactions are committed.

If I click No, the transactions aren't committed.

After I close the tab, my locked table will be released, then I can query successfully.

begin try 
    -- some process
    begin transaction
    update ...
    output ...
    

    insert ...

    -- I missing this commit statement below
    commit transaction  
end try
begin catch 
    if (xact_state()) = -1
    begin 
        rollback transaction;
        ;throw
    end;

    -- this statement I want to compare to 1, but mistake write to -1, but since the throw statement let the mistake can't be triggerd
    if (xact_state()) = 1
    begin
        commit transaction;
    end;
end catch;

Answer 10

The behaviour is not defined, so you must explicit set a commit or a rollback:

http://docs.oracle.com/cd/B10500_01/java.920/a96654/basic.htm#1003303

"If auto-commit mode is disabled and you close the connection without explicitly committing or rolling back your last changes, then an implicit COMMIT operation is executed."

Hsqldb makes a rollback

con.setAutoCommit(false);
stmt.executeUpdate("insert into USER values ('" +  insertedUserId + "','Anton','Alaf')");
con.close();

result is

2011-11-14 14:20:22,519 main INFO [SqlAutoCommitExample:55] [AutoCommit enabled = false] 2011-11-14 14:20:22,546 main INFO [SqlAutoCommitExample:65] [Found 0# users in database]